A carbon steel pipe (STPG) is used as a steel pipe for pressure piping. The carbon steel pipe has a minimum yield load and a breaking strength determined by the Japanese Industrial Standards (JIS). On the other hand, a coupling joint for connecting pressure pipes requires the yield load and the breaking strength exceeding those of the pressure pipes. Specifically, a tension test sets a joint strength to such strength that the connected pressure pipe base material should be cut. In addition, in the welding connection, the pressure pipe is required to extend a leg length on the entire circumference in order to keep such weld strength that the pressure pipe base material should be cut in the same way. Such a welding work requires skill, and therefore, the person who can perform the work is limited.
The connecting means without welding is means for adhesive or bolts. For example, both of patent literatures 1 and 2 disclose means for connecting pipe and pipe by bolts. These technologies are to fix the pipes, wherein electric wires are housed, by deforming the pipe side by fastening force of the bolts screwed on peripheral walls of the pipes along their radial direction. These pipes are to protect the inner electric wires, and the electric wires in themselves do not either have pressure or leak. Patent literature 3 discloses a technology for connecting poles by the bolts, wherein the poles are fixed by deforming the side surface by fastening force of the bolts.
On the other hand, the applicant previously has proposed a pressure pipe connection technology for guiding high-pressure fluid without using welding, wherein bolts are used as connecting means as shown in patent literature 4. There are provided a plurality of non-penetrating holes on the pressure pipes, and the pressure pipes are fixed by the bolts from a surface of a sleeve for externally fitting the pressure pipes towards the non-penetrating holes. The non-penetrating holes provided on the pressure pipe reduces the breaking strength of the pressure pipe itself. The patent literature 4 discloses that the non-penetrating holes are arranged on the pressure pipe at equal angular intervals on the entire circumference so as not to be overlapped to each other in the direction of the center line to disperse stresses in order to make the strength of the connecting portion exceed the yield load required for the pressure pipe (when the pressure pipe is STPG370, the value specified by standards is 370 N/mm2).